The invention relates to a downhole tool to generate tension signals on a slickline.
Certain downhole oilfield applications, such as perforating applications, require the ability to be able to position a tool at a particular and known spot in the well. For example, a slickline service uses a slickline tool assembly that is lowered downhole via a slickline. A depth counter may be used to track the length of the dispensed slickline to approximate the depth of the slickline tool assembly. However, because the depth counter does not precisely indicate the depth, other techniques may be used.
For example, a more precise technique may use a depth control log (a gamma ray log, for example), a log that is run while drilling the well and indicates the depths of various casing collars of the well. In this manner, the slickline tool assembly may be run downhole and include a detection device to detect casing collars. When the detection device indicates detection of a casing collar, the coarse depth that is provided by the depth counter may be used to locate the corresponding casing collar on the depth control log. Because the depth control log precisely shows the depth of the detected casing collar, the precise depth of the tool assembly may be determined. From this determination, an error compensation factor may be derived. Then, when a perforating gun is lowered downhole, the error compensation factor is used to compensate the reading of the depth counter to determine the position of the gun. Unfortunately, the error may not be the same, because more or less line may be dispensed than was dispensed when the error compensation factor was derived. Thus, more strain on the line may cause the compensation that is provided by the error compensation factor to be inaccurate.
The slickline tool assembly does not have the benefit of electrical communication with the surface of the well. Instead, the slickline tool assembly may generate tension pulses on the slickline to indicate the detection of a casing collar. To accomplish this, the conventional slickline tool assembly may perform some sort of physical interaction with a well casing. For example, the slickline tool assembly may include a mechanical drag device to generate the tension pulses. As an example, the mechanical drag device may be an end of a tubing locator, a device that includes a set of arms that extend to make contact with the well casing when the tool initially passes the end of the tubing of a mule shoe. In this manner, when the slickline operator attempts to pull back into the tubing, the arms catch on the restriction and do not close until a certain amount of tension is applied to the end of the tubing. This catch and release sequence creates a tension pulse on the slickline. This technique may be risky if the end of tubing locator does not release, a condition that may cause the tool assembly to become lodged in the well. Furthermore, the well may not have a suitable profile to permit proper operation of the end of tubing locator.
Thus, there is a continuing need for an arrangement that allows real time depth indication at multiple points while running a particular downhole tool (a perforating gun, for example).
In an embodiment of the invention, a downhole tool that is connectable to a line to be run downhole with the tool includes a housing, a sensor and a mechanism that is located inside the housing. The mechanism is coupled to the sensor to, in response to the detection of the feature by the sensor, generate a tension signal in the line without physically contacting a downhole structure.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.